It is known that conventional flowmeters with resistor heaters have various measurement errors. One of those measurement errors is a temperature characteristic error that occurs upon a detecting device, such as a resistor heater, heated by heat transmitted through a structure of the flowmeter with the resistor heater. Heat generating sources typically include 1) an engine and an exhaust pipe, and 2) a power transistor forming a signal amplification circuit in an electronic circuit section of the flowmeter with the resistor heater. There are possibly two heat transmission routes, i.e., A) one along which heat is transmitted through the structure of the flowmeter with the resistor heater and then directly reaches the detector, and B) the other along which heat is transmitted through the structure of the flowmeter with the resistor heater to raise the temperature of an auxiliary passage wall, whereby the temperature of an airflow in contact with the auxiliary passage wall rises and resulted heat reaches the detecting device.
When heat is transmitted to a temperature sensor for measuring an air temperature, the temperature detected by the temperature sensor is higher than the ambient air temperature by the amount of heat transmitted to the temperature sensor, and the amount of the transmitted heat directly produces an error of the measured temperature. When heat is transmitted to a resistor heater, the necessity of electrically heating the resistor heater is reduced in the amount of heat detected by the detecting device, and the output of the flowmeter with the resistor heater is reduced correspondingly. This is because the flowmeter with the resistor heater operates such that the temperature of the resistor heater is controlled to be always held at a fixed value higher than the temperature of a heat-sensitive resistor at all times and an electric power required for that control is taken out as a measured value. When heat is transmitted to the heat-sensitive resistor, the amount by which the resistor heater must be heated is increased in the amount of heat received by the detecting device, and the output of the flowmeter with the resistor heater is increased correspondingly. In such a way, heat-induced errors occur in the flowmeters with the resistor heaters.
In the conventional flowmeters with resistor heaters, it is known to reduce or adjust the thermal effect upon the detecting device, such as the resistor heater, through structural members as disclosed in, e.g., JP, A 60-36916. Further, in the flowmeter disclosed in JP, A 60-36916, the shape and the material of a terminal supporting the detecting device are changed for adjustment of the thermal effect from the structural members.